This invention relates to multiplexed data communication systems and more particularly to minimizing the effects of cross-talk in such systems.
Electromagnetic transmission media such as wire, radio, microwave, etc. have long been used for implementing communication channels. The amount of information that can be passed through such media is a function of its bandwidth. With the recent use of communications for remote computing, control, data collection, etc. the available bandwidth of electromagnetic transmission media is the subject of increasing contention from would-be communicators. Such contention has led to interest in spectrally efficient modulation and multiplexing techniques. These are techniques which allow the reliable communication of the greatest number of bits per Hertz. Two examples of techniques which have considerable attention are now discussed.
Many investigations have been carried out dealing with the close packing of Frequency Division Multiple Access (FDMA) communicators. This work, has been concerned with development of modulation and windowing techniques which minimize cross-talk among FDMA communicators. Its philosophy lies in pre-transmission processing. Waveform distortion is purposefully introduced to reduce spectral side lobes. Unfortunately, from the view of actual implementation, this technique is not attractive. If the channel itself introduces even limited distortion of communication signals, the demodulation process is seriously affected. The FDMA procedure is still used and wasteful guardbands are still present. Finally, minimization of cross-talk requires careful synchronization of FDMA carriers which is often difficult to effect.
Along another line, there has been a significant effort invested in achieving greater spectral efficiency through statistical multiplexing techniques such as packets communications. However, this approach to improving efficiency is limited only to bursty type communications.